1. Field of the Invention
The present invention relates to a volume hologram transfer foil used for, for example, security application and the like.
2. Description of the Related Art
Conventionally, in most hologram transfer foils, surface relief type hologram having irregularity on the surface is used, and in its general structure, a substrate, a delaminating layer, surface relief type hologram layer, reflective film layer and heat sensitive adhesive layer are laminated sequentially. In the case of a hologram transfer foil, since layers such as a delaminating layer, hologram layer, heat sensitive adhesive layer and the like other than the substrate are cut due to a heat mold in its heat transfer (also referred to simply as transfer, hot stamp), it is necessary that a foil cutting property is excellent and hologram is assuredly transferred to a transferred body. A hologram transfer foil of general relief type shows an excellent foil cutting property and used in many applications.
On the other hand, as materials capable of being used regarding volume hologram, silver salt materials, photopolymers and the like are mentioned, and in general, photopolymers which can be produced by a dry process are used in mass production. However, such a photopolymer is required to have increased refractive index modulation value (Δn) to make the image of hologram brighter, and in allowing a film to have low viscosity and forming volume hologram, monomers and the like are moved in the layer, therefore, the hardness of a coated film itself is low. Consequently, a foil cutting property is poor and a transferring property is instable, leading to a problem that it is extremely difficult to make a transfer foil of volume hologram.
Even if the hardness of a material is increased by improving the volume hologram material itself for solving this problem, there is also a problem that it is difficult to completely improve a transferring property and refractive index cannot be modulated sufficiently by further increasing the hardness of a coated film, making the hologram darker.
Further, the one is known which the foil cutting property is improved by previously making slits on a delaminating layer, hologram layer, heat sensitive adhesive layer and the like at transferring portions and delaminating them from a slit line along a heat mold in transferring. However, this method has a problem that a slit mold for making slits and a slit process are necessary, needing time and cost.
Furthermore, the present inventors have suggested a volume hologram transfer foil having a hologram layer made of a specific material having specific breaking strain(Japanese Patent Application Laid-Open (JP-A) No. 2000-272295)). However, also in this method, soft thermoplastic resins are often used as a heat sensitive adhesive layer in making a transfer foil, consequently, its foil cutting property is a problem in some